The present invention relates to a circuit with a selectable phase output signal, and more particularly to such a circuit for use in a television stereo decoder.
In the United States it has been proposed to transmit a television signal having stereo audio components. In one proposed system, known as the "Zenith" system, an L-R signal representing the difference between left (L) and right (R) stereo signals is transmitted as an amplitude modulated (AM) suppressed carrier at twice the horizontal scanning frequency. To maintain compatibility with existing monophonic (mono) receivers, a monaural signal, consisting of an L+R sum signal, is transmitted as a frequency modulated (FM) carrier in the conventional fashion. In a television receiver, the L+R and L-R signals are demodulated and then matrixed (added) to produce L and R signals. It has also been proposed to transmit an audio signal for a so called "second audio program" (SAP), e.g., for providing the main program in a second language. In the "Zenith" system, a frequency modulated carrier at five times the horizontal frequency is used for the SAP. For a more complete description of the Zenith system see pages 110-111 of "Television Multichannel Sound", Vol IA, published by the National Association of Broadcaster, Nov. 9, 1983; and also U.S. Pat. No. 4,339,772.
For both the L-R and SAP signals it is also proposed to use a so called "dbx" companding system in which variable preemphasis (compression) of high frequency components at the transmitter and complementary deemphasis (expansion) at the receiver is used in order to improve the signal-to-noise (S/N) ratio of the corresponding reproduced signals. The "dbx" companding system is described in detail in the publication entitled "Compandor Complexity Analyses" published by the Broadcast Television Systems Committee of the Electronic Industries Assocication (Supplement of Dec. 5, 1983). Basically, the purpose of providing substantial high frequency content in the transmitted signal is that it has been found that the greater the high frequency content of a transmitted signal the less perceptible will be noise (i.e., the more noise will be masked).
A companding system is not used for the L+R signal since, if it were, the ability of existing receivers to properly reproduce monaural signals would be disturbed. Furthermore, the transmitted L+R signal is less subject to being contaminated by interference than the higher frequency L-R or SAP signals.
For reasons of economy, it is desired to use the one expander for both the L-R and SAP signals. However, when this is done, it has been found that the SAP signal in one channel is out of phase with respect to the SAP signal in the other channel due to an inverter stage, which is required for stereo demultiplexing as will be explained below. The resulting sound emanating from the loudspeakers can be annoying to the listener.